A top coating composition for automobiles generally contains, as a film forming binder, a combination of a hydroxyl group containing polymer and a melamine curing agent. The cured film obtained from the melamine curing system, however, has poor acid resistance and is damaged by acid rain which has recently become one of serious problems. The damage of the film provides poor appearance, and a top coating composition having improved acid resistance has been demanded.
In order to overcome the above mentioned defects, the present inventors have proposed novel coating compositions without employing the melamine curing agent in Japanese Patent Kokai Publications 45577/1990, 287650/1991. The similar coating compositions are also disclosed in U.S. Pat. No. 2,977,334 (to Zopf et al.), 3,399,109 (to Zimmerman et al.) and 3,528,935 (to Marion et al.). The proposed coating composition cures by means of the reaction of acid with epoxy and therefore has good acid resistance and sufficient weather resistance for an automotive top coating application.
On the other hand, a high solid type of paint which contains decreased amount of solvent has been desired in recent years, for reducing a problem of air pollution. However, the prior art coating composition have a high viscosity due to their high functional group concentration and they have to contain fairly large proportion of solvent to attain a proper coating viscosity.
Japanese Patent Kokai Publication 64181/1990 discloses a resin composition for use in paint which contains a hydroxyl group containing polysiloxane and a cross-linking agent. Japanese Patent Kokai Publication 138228/1989 discloses a process for producing a hydroxyl group containing polysiloxane. However, the disclosed paints have too poor acid and mar resistance for use as an automotive top coating.
It is therefore desired to develop the top coating composition which has an excellent acid and mar resistance, and a decreased solvent content.